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Tuesday, 26 January 2010

Mobile Phone Batteries: Past, Present and Future

Forty-five years ago, when, to most people at least, chips only ever came with fish, a man called Gordon Moore wrote a paper in which he said the number of transistors that could be squeezed on to an integrated circuit doubles about every two years. Three years later, Moore co-founded Intel, whose computer chips have, to this day, developed almost exactly at the dizzying pace he predicted. Today, an Intel microprocessor boasts more than a billion transistors packed so densely that you could fit two million of the things on the full-stop at the end of this sentence. What became known as Moore's Law has driven exponential growth in the digital revolution – the more transistors you can pack into a circuit, the faster and more powerful its chips can run while remaining cheap. But the batteries keeping those circuits pinging are not digital and still work according to basic principles developed more than 200 years ago.

In the 1780s, Italian physicist Luigi Galvani discovered that a dead frog's leg would spring to life when he applied two pieces of metal. Galvani had created a crude circuit and the phenomenon was taken up by his friend, the aristocrat professor, Alessandro Volta. His voltaic pile swapped frogs for brine-soaked paper and pieces of metal for a stack of alternating zinc and copper disks. Volta had created the world's first modern battery.

A battery remains, by its simplest definition, a device that turns stored chemical energy into electrical energy. A chemical reaction takes place within a series of cells with negative and positive electrodes separated by conductive electrolyte. When you hook up the battery, positively charged ions "swim" from the negative to the positive electrode, prompting negatively charged electrons to power the bulb of a torch or the screen of your iPhone. It's a chemical process and, up to a certain point, you can't shrink chemistry. Peter Bruce, a professor of chemistry at the University of St Andrews, says that while computer performance has effectively doubled every two years, the energy density in batteries has increased five times in about 100 years. "If you want to store more energy you really have to develop new materials and new concepts," he says. "It's not just making the same things smaller."

Bruce is among a host of scientists racing to get more out of the modern battery. He owes a debt not only to Volta but also to the man whose work in the 1970s gave us the modern rechargeable battery that powers nearly all our gadgets. Stan Whittingham, a British-born American chemist who studied at Oxford in the 1960s, was working at the research division of the oil giant, Exxon, when he realised that the excellent energy-storing properties of the element lithium made it an ideal material to be used in rechargeable batteries. "A lithium-ion battery holds about five times as much energy as a lead one," Whittingham says on the phone from Binghamton University in New York, where he's a professor of chemistry. "It got a lot of people excited because it was really a technology-changing idea. Without lithium-ion batteries, you wouldn't have your iPod or your mobile phone. They've given us so much but of course people want more and more."

Bruce is taking up that challenge with his "air-fuelled" rechargeable lithium battery. Put very simply, the Stair cell (St Andrews air cell) uses nothing more complicated than air as a reagent in a battery instead of costly chemicals. By freeing up space and exploiting one of the few elements that is free, Bruce's cells can squeeze more power into a smaller space at a reduced cost. "By using air in the cell we can get much higher energy storage up to a factor of 10," Bruce says. "That's exciting because it's difficult to improve the lithium ion battery beyond a factor of two."

A battery with 10 times the storage of the one powering your phone would see a return to the days of weekly phone charging. Meanwhile, other scientists are working to solve that other great problem of the modern battery – the time it takes to recharge. Gerbrand Ceder at the Massachusetts Institute of Technology (MIT) has been looking at improving the way the lithium ions themselves move through batteries – the faster they "swim", the more quickly they charge the battery. Ceder and his team manipulated the materials inside batteries to make the ions' passage smoother and watched as they travelled at incredible speeds. Ceder estimates that a prototype battery made using the process could be charged not in hours or even minutes but seconds. "If we could cut charging time from, say, two hours to one hour, you would probably still do it overnight," he says. "But if it's one minute, you would stand by and wait – it would be like filling your car or getting a cup of coffee."

Ceder has also worked with a team that has used genetically-engineered viruses to build the positively and negatively charged ends of a lithium-ion battery. The new batteries would be more flexible and efficient than existing technology but, like MIT's fast-charging battery and Bruce's Stair cell, they are very much on the laboratory drawing board.

It's a measure of both the greatness of the modern battery and the challenges faced by developers that, as Whittingham puts it (perhaps with a degree of pride): "For the next five years at least it's just lithium." In the meantime, manufacturers are racing to launch energy-efficient screens and hardware that place less demand on batteries. But with so much riding on the next big breakthrough, it's only a matter of time before we get batteries fit to power the next generation of gadgets and cars. For those of us increasingly shackled to our phone chargers, that time can't come soon enough.

3 comments:

The future of mobile phones will definitely be in the paper thin batteries. The new type of battery that can be printed on paper, and can be used for much longer than standard batteries, has also been called the Silver Ink or Silver Bullet from Xerox.